The ability to scan objects optically is useful in many applications. By ‘scanning optically’ we mean to illuminate and/or detect radiation at/from a point on an object while scanning the point in one or more directions across the object.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Typically, the position measurement is captured by moving the substrate and the alignment sensor relative to one another, without the substrate and sensor having to be brought to a stop. The position of the mark relative to the (known) position of the alignment sensor is measured optically, by scanning the marks with an optical spot. (It does not matter whether the sensor moves while the substrate is stationary, or only the substrate moves, or both move.) The alignment sensors should scan the marks as quickly as possible for optimum throughput, but this need for speed places limits on the accuracy of the position measurements that can be obtained. It is inherently more difficult to acquire a position accurately in a short time. In addition, a measurement acquired in a short time, even if the measurement itself is perfectly accurate, will be susceptible to disturbance by dynamic positioning errors (vibrations, servo errors etc.) in the positioning of the substrate and alignment sensor.
To increase the acquisition time, it is recognized that an optical scanning of the spot additional to the scanning of the sensor itself could be useful. However, conventional optical scanning mechanisms would add cost and bulk to the sensor, and/or lead to inaccuracies in the measurement, defeating the purpose of the optical scanning.